Explainer: what is biological classification?

Author

Disclosure statement

Gerry Cassis receives funding from the Australian Biological Resources Study. He is a consultant for the Australian Atlas of Living Australia project. He is the President of the International Heteropterists Society.

For taxonomists, days often start – and sometimes end – with the question: what’s that?

Whether you’re an entomologist, staring at a new species of riffle bug from a rainforest stream, or a paleontologist piecing together the remnant lifelines of a trilobite, the correct placement of a species in the Tree of Life is a universal and most fundamental task.

This job of assigning species into a biological classification is the science of taxonomy – sometimes also called systematics.

In the biological sciences, taxonomy has been the bedrock of our work for over 250 years. In 1758, Carolus Linnaeus, the Swedish naturalist and founder of biological classification, published his 10th edition of the Systema Naturae.

Carolus Linnaeus was the founder of taxonomy.Wikimedia Commons

This book of names is the commencement date for classification. No names before this date count today.

Just as many of us have double-barrel names, so too do species, with each composed of a genus name followed by a species name.

For humans, Homo is our genus and sapiens our species. This is called binomial nomenclature – or two-part naming – and it has proven to be one of the more enduring information systems developed in recent centuries.

Of course, it doesn’t all end with the binomial Homo sapiens, as we belong with other Homo species, as well as our great ape cousins, to the family Hominidae.

In turn, hominids belong to the order Primates, gathering us with monkeys, lemurs, lorises and tarsiers.

Eventually, we are nested within the animal kingdom (Metazoa), and finally to all of the Tree of Life.

Peter Halasz/Wikimedia Commons

All of this is akin to the ultimate “Russian doll” set, where a hierarchy of relationships is established (see image left).

Herein lies the power of classifications because taxonomic hierarchies are highly informative and predictive, while at the same time being simple to understand.

The genus Homo tells us something about all of its species: they share characteristics that are known to them and only to them, and not to other genera such as their nearest relative, the fossil genus Australopithecus.

Biological classifications are “natural” because they reflect the evolutionary history of the organisms being classified.

They differ from so-called “unnatural” classifications, which are systems of convenience – think, for example, of how the nuts and bolts are arranged in a hardware store.

Biological classifications, in contrast, are based on recovering the relationships between species, so the species most closely related share attributes (e.g. physical features, behavior, DNA) that have been inherited from a common ancestor.

It may seem that the taxonomic task is straightforward. But this is far from the truth. Nature is not arranged into self-identifying units that are neatly packaged for us to just pick off.

It is a truism also that within the endless range of biological variation we scientists find ourselves confronted with, the patterns we discern must be attributable to evolutionary history.

But at the same time, the edges of our taxonomic units such as species are often fuzzy, and different lines of evidence can and often do give conflicting results.

Take for example the position of insects in the Tree of Life. For much of the second half of the 20th century they were considered most closely related to the centipedes and their relatives (myriapods).

Being land-adapted groups, they both possess a tubelike “respiratory” system, which was long held to be the ultimate indicator of their close relationship. This close relationship between insects and myriapods was long held to be bomb-proof.

New evidence from eye and brain anatomy and DNA sequences has, however, been combined to show insects are in fact close relative of prawns and their allies (that is, crustaceans).

When first proposed a few years ago, this view was received with a raised eyebrow, but is now widely accepted.

It means, in effect, that insect and myriapod “respiratory” systems have evolved independently, and the poor taxonomist was mislead by conflicting lines of evidence until eyes, brains and DNA sorted out the mess.

Such problems are commonplace in taxonomy and are caused by a process known as convergent evolution – a major cause of baldness amongst taxonomists!

From this example, you can see that taxonomy is beyond the mere naming of species. In fact, this is one of the last tasks in the exercise, and can be seen as a housekeeping exercise, albeit an important one.

The insect class has caused headaches for taxonomists in the past.Bugboy52.40/Wikimedia Commons

The taxonomist has the task of discovering species, through surveys and inventories, followed by investigating their features, inside and out, from the whole body to the gene.

Once a decision has been made about the uniqueness of a species or higher group (or taxon), and that it warrants a name of its own, its position within the Tree of Life must be decided (that is, its phylogeny).

Today, there are about 2 million described species of organisms. Yet, scientists estimate there are almost 10 million that probably exist in nature.

When you couple this with the suggestion that over 99% of all species thought to have existed are now extinct, one can begin to comprehend the enormity of the task facing taxonomists.

Taxonomy is essentially a comparative science. It is not driven by experiments but is concerned with examining and analysing the same or like features across multiple species.

Of the estimated 10 million species in nature, we’ve only describe roughly 2 million. CLICK FOR LARGER VIEW.KVDP/Wikimedia Commons

Although some scientists outside the discipline see it as descriptive and anachronistic, it is in fact one of the most adaptive of fields, and is data-hungry like few others.

Taxonomists quickly adopted the techniques and findings of the molecular revolution, beginning from the 1960s onwards, as a new source of data for reconstructing relationships and evolutionary history.

Despite the earlier, unbridled, enthusiasm expressed for genetic classifications, it is today seen as another line of evidence, but not a panacea or replacing comparative anatomy.

Taxonomists today face a daunting task at a time when species are being lost at an alarming rate and our knowledge of life on Earth is still cursory.

Yet, despite its more than 250 year history, the goals of taxonomy remain one of the greatest research programs in science today – to reconstruct the Tree of Life in all its magnificent detail and before it disappears!